This study longitudinally examined the effects of cognitive and sociopsychological individual differences (aptitude, motivation, personality) and the quantity and quality of second language (L2) experience on L2 speech gains in naturalistic settings. We elicited L2 spontaneous speech from 50 Chinese learners of English at the beginning and the end of their first 4 months of study abroad. Then, we linked the participants' gains in comprehensibility (ease of understanding) and accentedness (linguistic nativelikeness) to their individual difference and experience profiles. The participants' gains in comprehensibility were associated mainly with the amount of their interaction with fluent English speakers during immersion and secondarily with certain cognitive (grammatical inferencing) and sociopsychological (extraversion) individual differences. Furthermore, the amount of interactive L2 use mediated the effect of sociopsychological individual differences (extraversion and potentially ideal L2 self). In contrast, gains in accentedness tended to be less subject to experience effects but could be affected by certain pronunciation-related cognitive individual differences (phonemic coding).